Thunderbolt to FW adapter worth it ?

I have an external 2Tb drive (WD Studio Mac edition with metal casing, USB 2 and FW800).
I will be buying a MBA with 256Gb SSD. I won't keep my entire iphoto library on my MBA (80+ Gb) so I would point iphoto to my library on the external drive (which I have backed up on another drive).
Now, I can just access the WD through USB2 or I could fork out for a TB to FW adapter.
Question is whether I will get the full FW800 speed I'm enjoying right now on my iMac (which has a true FW800 port). Or will be ok with just USB 2 ?
Would USB2 be fast enough to run Wow from it ?

Of course, if I decide to connect an external display to the MBA, I'd be out of TB ports...

FW800 is twice as fast as USB 2.0, thus if you use the external HDD often, the 29 USD for the TB <> FW800 adapter might be more than worth it.
USB 2.0 might suffice though, but running WoW from it, can be taxing. As you have a 256 GB SSD in your new MBA, WoW should have enough room to live on that, unless WoW is 100 or more GB in size (I do not know that, though my Applications folder on my 256 GB SSD is 50 or so GB big).

I believe the Wow folder is about 35Gb. I can of course install it on the SSD but I've got heaps of other games on Steam (albeit older games, they will run fine on the MBA but take up room nonetheless). And Steam doesn't like running games from an external drive...

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